fix 2 error messages

src/nodes.lua

@@ -66,7 +66,7 @@ end
 	
 	--Christmas easter egg
 	minetest.register_on_mapgen_init( function()
-		if skins then
+		if rawget(_G, "skins") then
 			skins.add("character_snow_man")
 		end
 	end

src/sled.lua

@@ -68,7 +68,7 @@ local sled = {
 	visual = "mesh",
 	mesh = "sled.x",
 	textures = {"sled.png"},
-	HUD,
+	nil,
 	
 	driver = nil,
 	sliding = false,